Implement invoice model and admin API

gnuviechadmin/invoices/admin.py

@@ -0,0 +1,14 @@
+from django.contrib import admin
+
+from invoices.models import Invoice
+
+
+class InvoiceAdmin(admin.ModelAdmin):
+    list_display = ["invoice_number", "customer"]
+    readonly_fields = ["customer"]
+
+    def has_add_permission(self, request):
+        return False
+
+
+admin.site.register(Invoice, InvoiceAdmin)